You don't need a crystal having a specific frequency for communicating via serial port. A 4Mhz or 20Mhz crystal will do, as long as you adjust the values in the baud rate generator registers of the PIC accordingly.
You need to use an ADC for converting the output of the photocell to a digital value (if the PIC you're using doesn't have one of course)
The other thing you need is a level shifter like MAX232 to convert the voltage levels between the PIC and the serial port of your PC.
The crystal has nothing to do with the thing you want to accomplish.